THE METABOLISM 01 THJ I IBBOHYDRATEfi 661 



combines with glucose to form it is believed to be related to the internal 

 secretion of the pancreas (see page 676 . 



The difficulty in explaining why the glucose of the blood does uol con- 

 stantly leak into the kidnej is, however, the only evidence upon which the 

 hypothesis of a blood-sugar compound rests. No chemical evidence can 

 be offered in supporl of such ;i view. On the contrary, all experimental 

 work indicates that the sugar exists in a free state; but unfortunately even 

 this evidence is uot convincing. Thus, it has been found that, when Bpeci 

 mens of perfectly fresh blood are placed in a series of dialyzei sus- 

 pended in isotonic saline solutions, each solution containing a slightly dif- 

 ferent percentage of glucose, diffusion of glucose, in oi r other direction, 



occurs in all of them Bave one namely, that in which the percentage of 

 glucose in the fluid outside the dialyzer is exactly equal to t lie total sugar 



content of the blood. Such a result can I splained only by assuming that 



all of the sugar in the blood exists in a freely diffusible state. In its general 

 nature this experiment is analogous to that by which the tension or partial 

 pressure of CO a is determined in blood (see page 338 . 



It has been assumed by many clinicians thai glycosuria may sometimes 

 become developed because the kidney fails to hold hack the blood sugar 

 even when the percentage is not above the normal- so-called renal dia- 

 betes. For the diagnosis of this condition a comparison must be made be- 

 tween the sugar concentration of the blood and that of the urine. In order 

 to do this a1 least tWO samples of blond must be taken, one of them at the 



beginning and the other at the end of a period during which urine is being 

 collected. Merely to tind thai one sample of blood collected before or after 

 or during the period of urine collection contains a normal percenl g 

 sugar, docs not necessarily indicate that at some other period while the 

 urine was being produced a temporary hyperglycemia may not have ex- 

 isted. 



